1. Field of the Invention
This invention relates to electronic systems for detecting the unauthorized passage of protected articles through an egress passageway and more particularly it concerns novel means in such systems for providing electronically monitored egress passageways of substantially unlimited width.
2. Description of the Prior Art
In U.S. Pat. No. 3,500,373 there is disclosed an electronic theft detection system for protecting articles of merchandise in a retail store. Each article to be protected is provided with a tag or label which contains a concealed resonant electrical circuit. Transmitter and receiver antennas are provided at an egress facility such as a doorway and the transmitter antenna is energized to generate an electromagnetic field in the vicinity of the doorway which varies cyclically in frequency, e.g., the frequency may shift over a range from 0.8 to 1.2 megahertz at a rate of 500 hertz. When a protected article, carrying a concealed resonant circuit tuned to resonate at a frequency within the sweep range, is carried into the electromagnetic field, it reacts with the field and produces a characteristic response. The exit region is continuously monitored for the occurrence of this distinctive response and when it is detected an alarm is sounded.
U.S. Pats. No. 3,696,379, No. 3,868,669 and No. 4,016,553 and copending U.S. Pat. application Ser. No. 697,128 filed June 17, 1976 show various additional features, adaptations and improvements to the basic system of U.S. Pat. No. 3,500,373.
U.S. Pat. No. 3,493,955 shows an electronic theft detection system which utilizes an electronic transponder circuit as a target on protected articles. This circuit responds to an electromagnetic interrogation signal at one frequency and retransmits at another frequency. Transmitter antennas are provided on the floor and one side of an egress passageway and a receiver antenna is provided on the opposite side of the passageway.
Copending U.S. Pat. application Ser. No. 715,568 filed Aug. 18, 1976 shows an electronic theft detection system which operates on a different principle, namely the detection of target strips of a saturable magnetic material such as permalloy. This is accomplished by producing an alternating magnetic interrogation field at a doorway and then detecting other fields which the target strips produce at harmonic frequencies when exposed to the interrogation field at the doorway. This magnetic type detection system uses transmitter and receiver coils in the form of partially overlapped loops on opposite sides of the doorway.
French Pat. No. 763,681 to P. A. Picard shows a similar detection system and in one embodiment there is shown a balanced receiver antenna comprising a double loop in the form of a figure eight. The afoementioned U.S. Pat. No. 4,016,553 also employs a balanced receiver antenna in the detection of resonant electrical circuits.
The theft detection systems of the prior art all utilize an antenna or field generating means, either a combined transmitter and receiver coil including a loop which extends around an egress passageway or separate transmitter and receiver antennas on opposite sides of the egress passageway. This field generating means provides adequate signal energy levels across the passageway but they do limit the effective width of the passageway. In general, the systems which employed resonant electrical circuits to be detected had passageway widths of about thirty three inches (84 cm.) while the systems which employed saturable magnetic strips to be detected had passageway widths of about thirty inches (76 cm.).
In modern merchandising operations such as in shopping malls, it is often desired to provide store entrances with very wide and unobstructed openings. This facilitates movement of customers into and out from the store and it provides a less intimidating and a more inviting appearance than a conventional doorway provides. Such large passageways, however, are not suited to the theft detection systems of the prior art because the antenna arrangements of those systems caused confinement of the width of the egress passageway and thus conflicted with the concept of a wide and unobstructed opening.